Injection Mold Design

Missing small

Most of the products we design are now a days made of plastics and majority of the plastics parts are only injection molded.

So why not we understand the injection molding process and know about the injection mold design before we attempt to design products.

(Any comments, suggestions and questions are always welcome.)

Murali. K
(Injection Mold Designer)

  1. Step 1: Injection Mold Design

    Before we move into injection mold design: belonging to the design community let us first question our selves.

    What is design?

    Simply to say Design is a detailed pre-plan of how a product may appear after manufacture and / or assembly. So a product well planned or well designed is half done.

    (Keep in mind: Failing to plan is planning to fail).

    Now knowing how important is design (50% of a products success) & being a designer of some product let us jump to injection molding and injection mold design.

  2. Step 2: Injection Molding

    To start with injection mold design it is important we understand the Injection molding process first.

    Injection molding is a process meant for mainly thermoplastics material.

    Thermoplastic is a material that melts and flows on the application of heat and/or pressure.

    Injection Molding:

    Injection molding is a process in which molten plastics material is injected at high pressures into an injection mold which contains the impression (required shape) of the component/components to be produced.

    Once injected the material is then cooled in the mold. Mold is then opened and the component (/s) is (/are) ejected to obtain the required part.

  3. Step 3: Part Design for Injection Mold

    In the previous session we understood that the plastics material is heated and injected into the plastics mold.

    1. The over all wall thickness of the part should be such that it lets the molten material reach the part extremities.
    2. This basic wall thickness of a part is entirely material dependent. Thus different materials would require a part to be designed with different wall thicknesses. (Melt flow indexes of different materials vary)
    3. The over all wall thickness of the part should also be such that it allows the part to be cooled to a solid, rigid state for ejecting it from the mold.

    (Keep in mind the outer walls of the part in contact with the steel surface of the mold cools first forming an insulating layer around the inner molten plastic. Further as the inner plastic cools the outer cool layers sink in-words leading to surface defects like sink marks)

    4. Hence it is important to design a part with optimal wall thickness based on the requirement.

    5. If we consider any tube type component, material is injected at high pressures is contained between the core and cavity halves.

    Further as the material cools the plastics material shrink a bit and tends to stick tightly on the core part of the mold. (Therefore component generally stays on the core part of the mold.)

    So to facilitate the core and cavity halves to be separated and the smoothly eject the component out of the core enough draft (angle) should be provided on the product side walls.

    (Although there are many other points to be considered for designing a plastics product the above three points are important and obvious)

  4. Step 4: Injection Mold

    What is a Plastics Injection Mold?

    A plastics injection mold is a rigid assembly of core and cavity halves making an impression of the component required, into which molten plastics material is injected, cooled and ejected to obtain the required component.

    Any successful mold designer should strictly look at six mold systems (apart from concentrating on building rigid mold parts)  viz. 

    1. Guiding system

    2. Feeding system

    3. venting system

    4. cooling system

    5. Ejection System

    6. Control System.

    Let us know each of them in detail in coming sessions.


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